DC bias voltage circuits are well-known and utilized in a variety of applications to supply a desired DC voltage to an electronic circuit or to a transducer such as a capacitive microphone. In many of these applications, the desired DC voltage is required to possess a very low noise level to prevent that noise from being injected into signals generated by the electronic circuit or the transducer. The noise of the DC bias voltage circuit can include thermal noise and/or voltage ripple or clock signal residues of a DC-DC converter or charge pump forming part of the DC bias voltage circuit. DC-DC converters or charge pumps are often required to generate an appropriate electrical DC bias voltage to a capacitive transducer from a power supply source. The DC bias voltage is required in order to appropriately convert audible sound, or infrasonic or ultra-sonic sound, into a corresponding electrical signal. In particular, in order to guarantee a level of performance sufficient for common capacitive microphone applications, a high DC voltage is often required for example between 4 V and 20 V.
To reduce the noise level at the output of the DC bias voltage circuit low-pass filters have been used in the prior art where a RC based low-pass filter have been coupled to an output of a charge pump stage to attenuate noise at the output thereof. However, in order for the low-pass filtering to be effective the filter must have a very low cut-off frequency for example below 10 Hz. Such low cut-off frequencies inherently create a long power-up time for the DC bias voltage circuit due to the settling time of the low-pass filter being substantially inversely proportional to the cut-off frequency. Furthermore, to produce such a low cut-off frequency, the filter resistor must have a resistance of extremely high value for example a value above several GΩ or even tens of GΩ. Unfortunately, it is not possible, in integrated-circuit technology, to obtain resistors with such high resistance values without an unacceptable area consumption of the semiconductor substrate or die. This short-coming has been addressed in the prior art by replacing the high value resistor with non-linear devices, for example a pair of diodes in anti-parallel configuration, capable of providing the high values of resistance required within acceptable die area constraints. However, the actual resistance or impedance values realized by such non-linear devices tend to vary widely across operational temperature, semiconductor process outcome and bias conditions. Consequently, the power-up time of the DC bias voltage circuit becomes ill-defined which makes it difficult during manufacturing testing to estimate the point in time where the circuit under test has reached settled or nominal operation. Likewise, powering-up time becomes uncertain when normal operation is required.
In addition, the long settling time of the low-pass filter causes an undesirable prolongation of manufacturing test time during manufacturing testing of the DC bias voltage circuit, or manufacturing test of a condenser microphone assembly incorporating the DC bias voltage circuit as the bias voltage source of a condenser element. The performance characteristics of the DC bias voltage circuit or the performance characteristics of the condenser microphone assembly, such as electroacoustic sensitivity and noise level, can only be appropriately measured after the DC output voltage of the DC bias voltage circuit is settled. This is because microphone sensitivity is determined by the DC voltage across the condenser element, typically comprising a deflectable diaphragm structure and stationary back plate structure, of the condenser microphone assembly. Hence quick and accurate settling of the DC bias voltage across the condenser microphone element upon power-up will allow tighter sensitivity specifications which are useful in numerous applications such as beamforming applications where tightly matched condenser microphone pairs, triplets etc. are used to provide fixed or adaptive directionality of sound pick-up.
U.S. 2010/0246859 describes a biasing circuit for an acoustic transducer. The biasing circuit is provided with actuable switches so as to connect a first terminal of an acoustic transducer to the biasing terminal of the voltage-booster stage, directly during a start-up step of the biasing circuit, and through filtering elements at the end of the start-up step.
Accordingly, there is a need in the art for the DC bias voltage circuits that are capable of rapid power-up without sacrificing noise performance during normal operation. Further advantages are provided if the DC bias voltage circuit also provides a well-defined power-up time across semiconductor process variations, operational temperature variations and can be implemented in integrated circuit technology with small die area consumption. The latter consideration is particularly important for cost sensitive applications like high-volume low-cost components for electronic consumer products such as miniature ECM and MEMS microphone assemblies for portable communication devices.